Non-Destructive Evaluation (NDE) methods are used to detect defects within articles without damaging the articles themselves. In particular, several methods are known for inspecting bonds such as welds between joined components. In such welds, defects may comprise unbonded regions or inclusions. One such method is ultrasonic inspection.
In ultrasonic inspection, an ultrasonic wave is produced by a transducer and transmitted through a medium to the article to be inspected. In a single sided inspection, the transducer transmits an ultrasonic wave, which is then reflected back to the transducer by deformities or discontinuities in the structure of the article. These reflected signals can be used to indicate that a void is present within the bond, by for example comparing the amplitude of the transmitted and reflected waves.
In articles comprising anisotropic materials however, such as diffusion bonded titanium alloy, the diffusion bond interface can act as a weak reflector. This is because in anisotropic materials, the speed of sound in the material is dependent on the direction of travel of the sound wave relative to the crystallographic orientation of the material. In some materials, such as titanium alloy, the crystals often form macroscopic “colonies” of crystals having a similar orientation up to several millimeters in size. FIG. 1 shows a typical diffusion bond interface 12 in an article 10 comprising anisotropic Ti-6Al-4V alloy. The different shadings represent crystals having different crystallographic orientations. The bond interface 12 will act as a weak planar reflector due to the different crystallographic orientations above and below the bond interface 12. An ultrasonic wave 14 will therefore be at least partly reflected by the bond interface 12.
Consequently, an amplitude based single sided ultrasonic inspection of the bond interface 12 will result in a spurious indication of a bond defect. FIGS. 3a, 4a and 5a show amplitude based single sided ultrasonic inspections of first, 10a, second 10b and third 10c articles. In general, lightly shaded parts of FIGS. 3a-5a represent high amplitude return signals, which would generally be interpreted as defects. As can be seen, each of the inspected articles 10a, 10b, 10c would “fail” an amplitude based single sided ultrasonic inspection due to the perceived defects.
One method of overcoming this above problem is to use a two sided ultrasonic inspection method. Such a method is described in “Ultrasonic Non-destructive Evaluation of Titanium Diffusion Bonds” by K Milne et al, J Nondestruct Eval (2011) 30:225-236.
The method described in Milne et al comprises two single sided inspections performed from either side of the article at a plurality of inspection locations. The resulting reflected waveforms from each side are acquired using conventional ultrasonic inspection equipment. They need not be acquired simultaneously, but each inspection location must be known so that the corresponding pair of waveforms from each side from can be spatially related to the other at a corresponding location. These waveforms are then transferred to a suitable computer where spectral (Fourier) analysis is performed in order to establish the phases of the diffusion-bond signals within the waveforms performed from the first side and the diffusion-bond signals within waveforms performed from the second side. The two diffusion-bond signal phases acquired at each scan position are compared, and this comparison yields information about the quality of the bond. In the method described by Milne, phase differences of 180° are taken to be ‘natural’ differences that occur due to the acoustic impedance mismatch of the grain colonies either side of the bond. However, phase differences tending towards quadrature (90° or 270°) are indicative of a reduction in interfacial stiffness. A double-sided inspection can be performed on the principle that the ‘natural’ acoustic impedance mismatches are always asymmetric about the diffusion bond plane: i.e. if the phase is x° for the signal taken from the first side, then the phase of the signal taken from the second side will be x+180°. However, phase differences that result from unfavourable conditions at the diffusion bond (such as defects, lack of bonding or inclusions etc.) are always symmetric about the diffusion bond plane: the phase contribution from the unfavourable condition will be the same in the signals from both sides. We can therefore eliminate the ‘natural’ contribution, leaving only that which results from unfavourable conditions.
It should be noted that a captured waveform will contain several signals, one of which is the diffusion-bond signal. Other signals could for example comprise material anisotropies or bulk material defects. When performing an ultrasonic inspection, other signals can be considered to represent noise. It is therefore necessary to identify the diffusion bond signal within each waveform from each inspection location, so that the spectral analysis can be performed on the correct signal. One conventional method for identifying the diffusion bond signal from the captured waveform is known as “gating”. The amplitude of each waveform is monitored until a predetermined threshold amplitude is breached, usually the echo from the top surface nearest the transducer, since this is often the largest signal in a waveform. Once the predetermined threshold amplitude is breached a fixed delay (determined by the thickness of the first component 18 in the article 10) is added to the time of the breach. The waveform between this new point and the end of the predetermined period of time has elapsed is taken to be the diffusion bond signal. The method is then repeated for the waveform taken from the other side for each inspection location.
However, this method requires access to both sides of the article in order to obtain the waveforms, preferably normal to the plane of the diffusion bond 12. In many cases this is not possible—for example, where the article to be inspected comprises the compressor bladed ring (known in the art as a “bling”) of a gas turbine engine, as shown in FIG. 2. The two-sided inspection technique described above is also relatively inaccurate, since the above described method results in an inaccurate identification of the spatial location of the waveforms (i.e. in relation to the plane of the bond), and the depth of the diffusion bond signals within the respective waveforms. Consequently, some articles having insufficient bond strength will “pass” the prior two-sided inspection techniques, while some articles having sufficient bond strength will “fail”, i.e. the prior methods provide both “false positives” and “false negatives”.